Igto] WIEGAND—TRANSPIRATION 441 
where plants during their vegetative period are as a rule exposed to 
dry air for only a few hours each day, and where their activity is not 
interrupted by a warm dry period but by frost and cold. On the 
Alps the complete drying-up of plants by the sun is rare, but the dry 
winds and hot sun at times make a lessening of the transpiration very 
desirable. On other mountains of the same latitude in Europe and 
Asia many hairy plants occur. Indeed, the account given by KERNER 
is almost an exact picture of what we should expect from the point of 
view of the present experiments. On the slopes and in the pockets 
on the Alps, where vegetation exists, the soil is warmed in summer 
and there is probably an available supply of water throughout the 
vegetative period. 
Quite different is the condition in the arctic regions. Thick 
€vergreen leaves replace the hairy ones. KERNER says: “When 
hairy coverings are present they are restricted to the under surface, 
especially to that of rolled leaves. They are never found on plants 
of rocky slopes, but only on those of damp marshy ground, or by the 
side of water which is for a short time free from ice.”’ He believes 
that such coverings are not concerned with transpiration at all, 
and that the absence of hairy coverings in the arctic regions is due 
to the moisture in the soil and the consequent absence of danger of 
drying out. This explanation, however, seems insufficient. If it 
were true, why should we find thick, highly cutinized, xerophytic 
leaves on such plants as Diapensia, Empetrum, Vaccinium, ete. ? 
Our explanation now would be that the soil in the arctic regions is too 
cold for root absorption even in summer, since it remains frozen only 
a few inches below the surface. The water even if present is there- 
fore not readily available, and there is constant physiological dryness. 
These are exactly the conditions necessary to demand a cutinized 
rather than a hairy flora. 
Our evergreen leaves, such as those of Rhododendron and Kalmia, 
are heavily cutinized, not tomentose. According to our theory 
this would be owing to limited water supply in winter, which is 
true. Owing to the coldness of the soil and the inactivity of all the 
living cells of the plant, there is constant physiological dryness. 
Plants of the great plains, where the wind is excessive but the 
ground not extremely dry, are commonly silky, strigose, or tomentose. 
